1. Field of the Invention
This invention relates to methods and fluids used in treating a subterranean formation. In particular, the invention relates to the preparation and use of an environmentally favorable fluid for use in the removal of deposits containing paraffin and asphaltenes. Also in particular, the invention relates to methods of forming and using the fluids.
2. Description of the Related Art
Various types of fluids are used in operations related to the development and completion of wells that penetrate subterranean formations and to the production of gaseous and liquid hydrocarbons from natural reservoirs. These operations include drilling, perforating, fracturing, modifying the permeability, or controlling the production of sand or water. The fluids employed in these operations are known as drilling fluids, completion fluids, work-over fluids, packer fluids, fracturing fluids, stimulation fluids, conformance or permeability control fluids, wellbore cleanout fluids, gravel pack fluids, consolidation fluids, and the like, and are collectively referred to herein as well treatment fluids.
Over time, as oilfield services introduce these fluids to subterranean formations and as subterranean formations release hydrocarbons for processing, solid or partially solid organic deposits develop along the surfaces of the formation, the wellbore, the screen, the gravel pack, or other equipment used to process the subterranean formation.
Asphaltenes are heterocyclic unsaturated macromolecules consisting primarily of carbon, hydrogen, and minor components such as sulfur, oxygen, nitrogen, and various heavy metals. These high-molecular-weight components of crude oil are in equilibrium under normal reservoir conditions. As crude oil is produced, this equilibrium may be upset by a number of factors, such as CO2 and rich-gas injection, pH shift, pressure drop, shear, streaming potential through porous media and charge metal surfaces, leading to asphaltene deposition.
Asphaltene deposition can occur anywhere in the production life cycle: in the near-wellbore region including perforations, in the tubing, downhole and surface chokes, and surface flowlines. Predicting where asphaltene deposition might occur requires an understanding of the mechanisms for asphaltene deposition. The key causes are pressure decrease and injection of incompatible fluids in the reservoir rocks.
Asphaltene problems can significantly reduce well productivity, causing troublesome operational issues, damaging formations, and decreasing production. The most common asphaltene removal techniques use xylene or xylene mixtures which typically have undesirable health, safety, and environmental characteristics.
Also, paraffin accounts for a significant portion of a majority of crude oils that are greater than 20° API. Paraffin has a straight chain linear structure comprised entirely of carbon and hydrogen. The paraffin with molecules that are larger than C20H42 are the components that cause deposition or congealing oil in crude oil systems. Paraffin can deposit from formation pores to the pipeline that deliver oil to the refineries. The deposits vary in consistency from rock hard for the highest chain-length paraffin to very soft, mayonnaise-like congealing oil deposits. Paraffin (wax) is mostly found as a white, odorless, tasteless, waxy solid, with a typical melting point ranges from 47° C. to 64° C. (116.6° F. to 147.2° F.), and a density of around 0.9 g/cm3. It is insoluble in water, but soluble in ether, benzene, and certain esters.
Solvency has been one of the primary methods of removing these deposits. A number of factors can affect the removal of paraffin from production systems. Some of these factors are: solvent used, type of paraffin, quantity of paraffin, temperature and contact time. Any or all of these can help determine success or failure of a paraffin removal treatment.
Different solvents have different abilities to dissolve paraffin. Two general classes of solvents used in the oilfield to dissolve paraffin are aliphatic and aromatic. Common aliphatic solvent used in the oilfield are diesel, kerosene, and condensate. Aromatic solvent used are xylene and toluene. Among all the solvents, xylene and toluene are more effective than aliphatic solvents in removing most of the paraffin deposits. Traditionally, aromatic solvents such as xylene and toluene have been used to remove damaging organic deposits such as asphaltene and paraffin from wellbore tubulars and the formation matrix. However, governmental regulations on the usage, disposal and volatile emission-limits of aromatic solvents are becoming increasingly restrictive. Practically, the flammability, acute toxicity, and environmental contamination concerns have made their use less attractive.
Clearly, a replacement for aromatic solvents in the oilfield is needed that is more logistically and operationally efficient as well as more socially and environmentally responsible. A fluid that is effective, relatively environmentally favorable, and low cost is desirable.